The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to a gimbal-mounted plate for carrying wafers, in which edge effects are reduced by aligning a wafer-engaging surface of the wafer carrying plate with a wafer polisher-engaging surface of an active retainer ring.
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical polishing (CMP) operations on semiconductor wafers, such as those made from silicon and configured as disks of 200 mm or 300 mm in diameter. For ease of description, the term xe2x80x9cwaferxe2x80x9d is used below to describe and include such semiconductor wafers and other planar structures, or substrates, that are used to support electrical or electronic circuits.
Integrated circuit devices may be in the form of multi-level structures fabricated on such wafers. A transistor device may be formed at one level, and in subsequent levels interconnect metallization lines may be patterned and electrically connected to the transistor device to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials. As more metallization levels and associated dielectric layers are formed, there is an increased need to planarize the dielectric material, such as by performing CMP operations. Without such planarization, fabrication of additional metallization layers becomes substantially more difficult due to variations in the surface topography.
A CMP system typically includes a polishing station, such as a belt polisher, for polishing a selected surface of a wafer. In a typical CMP system, the wafer is mounted on a wafer-engaging surface of a carrier (carrier surface). The mounted wafer has a surface (wafer surface) exposed for contact with a polishing surface, e.g., of a polishing belt. The carrier and the wafer rotate in a direction of rotation. The CMP process may be achieved, for example, when the exposed rotating wafer surface and an exposed moving polishing surface are urged toward each other by a force, and when the exposed wafer surface and the exposed polishing surface move relative to each other. The carrier surface is said to define a carrier plane, the exposed wafer surface is said to define a wafer plane, and the exposed polishing surface in contact with the wafer plane is said to define a polishing plane.
In the past, the wafer carrier has been mounted on a spindle that provides rotation and polishing force for the carrier. To enable the wafer carrier to properly position the exposed wafer surface for desired contact with the exposed polishing surface, for example, a gimbal has been provided between the spindle and the wafer carrier. The gimbal allows the carrier plane to tilt relative to a spindle axis around which the wafer carrier rotation occurs. Such tilting allows the carrier plane to be parallel to the polishing plane of the belt. Generally, however, provision of the gimbal results in more mechanical structures between the carrier surface and a force sensor mounted on the spindle. As a result, there is more of an opportunity for friction in the mechanical structures to reduce the force sensed by the sensor.
Others have provided so-called active retainer rings that support the wafer against horizontal forces to retain the wafer on the carrier plate. However, the design of such active retainer rings has not appreciated an adverse feature of such active retainer rings. Thus, such design did not take into account a gimbal-like action of such active retainer rings. Such action of such retainer ring mounted on the carrier may be appreciated in terms of a retainer ring plane defined by an exposed surface of the retainer ring (the ring surface). Such design did not appreciate that a lack of guidance of such active retainer ring allows such retainer ring plane to be positioned axially offset from the wafer plane in response to forces, such as a horizontal force of the belt acting on the ring surface. The amount of the offset may be referred to as a reveal, and if the reveal is positive, the wafer plane is closer than the ring plane to the polishing plane of the belt. In general, a negative reveal is used to properly seat, or position, the wafer on the carrier surface prior to polishing.
As an example of the lack of guidance of such prior active retainer rings, the motor, such as a bladder, that drives such an active retainer ring relative to the wafer has been flexible and allowed the retainer ring plane to move in an uncontrolled manner relative to the carrier plane and relative to the wafer plane. This uncontrolled relative retainer ring-wafer carrier movement has allowed the retainer ring plane to tilt and become out-of-parallel with respect to both the carrier plane and the wafer plane. Unfortunately, in the tilted orientation, the retainer ring is not co-planar with the wafer plane. As a result, such tilting results in the value of the reveal being different at different angles along the circumference of the wafer and of the retainer ring, i.e., around the carrier axis of rotation. Such differences in the values of the reveal are undesirable because, for example, they are uncontrolled and have caused problems in CMP operations. The problems may be understood in terms of the edge of the wafer, which generally includes an annular portion of the wafer surface extending from the outer periphery of the wafer inwardly about 5 to 8 mm, for example. The problems in CMP polishing arise because the variation in the value of the reveal results in the vertical profile of the edge of the polished wafer having a different value for each different value of the reveal.
What is needed then, is a way of allowing the retainer ring to move relative to the wafer plane while limiting the movement of the retainer ring so as to avoid such tilting. What is also needed is a way to prevent the retainer ring plane from becoming out-of-parallel with respect to both the carrier plane and the wafer plane so that the retainer ring plane and the wafer plane may be aligned, i.e., co-planar. What is also needed are structure and methods of allowing the retainer ring to move relative to the wafer plane while avoiding relative movement that results in the value of the reveal being different at different angles of rotation of the wafer and the retainer ring on the carrier axis of rotation. In particular, currently there is an unmet need for structure and methods of providing a uniform profile of the edge of a wafer in CMP operations while retaining the advantages of retainer rings that are actively moved relative to the wafer plane.
Broadly speaking, the present invention fills these needs by providing CMP systems and methods which implement solutions to the above-described problems, wherein structure and methods are provided for allowing a retainer ring to move relative to a wafer plane while limiting the movement of the retainer ring so as to avoid such tilting that causes the retainer ring plane to become misaligned (i.e., out-of-parallel with respect to both the carrier plane and the wafer plane, or not co-planar with the wafer plane). In such systems and methods, the retainer ring may move relative to the wafer plane, but the relative movement is limited so that for polishing the wafer the retainer ring plane and the wafer plane may be co-planar. In particular, the direction of the relative movement is limited to a direction perpendicular to the wafer plane and the carrier plane, whereby the value of any desired reveal remains the same at different angles around the periphery of the wafer and of the retainer ring, i.e., around the carrier axis of rotation. Thus, the advantages of retainer rings that are actively moved relative to the wafer plane are retained without having the non-uniform reveal problem.
In one embodiment of the systems and methods of the present invention, a carrier plate is provided with a carrier surface to support a wafer. A retainer ring is mounted on and for movement relative to the carrier plate. A linear bearing arrangement is mounted between the carrier plate and the retainer ring. The arrangement is configured to limit the movement of the retainer ring relative to the carrier, wherein permitted movement keeps the retainer ring plane parallel to the wafer plane, or for polishing, co-planar with the wafer plane.
In another embodiment of the systems and methods of the present invention, an assembly including the carrier plate is provided with a gimbal to movably mount the carrier plate relative to a spindle housing. The spindle housing is mounted on a drive spindle. The gimbal allows the carrier plate to move so that the wafer plane may move and become co-planar with the polishing plane during the CMP operations. The retainer ring is mounted on and for movement relative to the carrier plate, and thus may also move relative to the wafer. However, the linear bearing arrangement constrains both such relative movements by permitting only movement of the retainer ring relative to the carrier plate along a path parallel to a central axis of the carrier plate.
In yet another embodiment of the systems and methods of the present invention, the linear bearing arrangement is provided as an array of separate linear bearing assemblies spaced around the wafer carrier.
In still another embodiment of the systems and methods of the present invention, the linear bearing arrangement is provided as an array of separate linear bearing assemblies in conjunction with the retainer ring, wherein a force applied to the retainer ring by the polishing belt is transferred to the carrier plate parallel to an axis of the carrier plate to facilitate calibration of the retainer ring.
In a related embodiment of the systems and methods of the present invention, the linear bearing arrangement is assembled with the retainer ring in conjunction with a motor for moving the retainer ring relative to the wafer mounted on the carrier so that an exposed surface of the wafer and a surface of the retainer ring to be engaged by the polishing pad are co-planar during the polishing operation.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.